The present invention is a process for smelting ferrosilicon alloy. The process comprises adding a carbon source and tailings comprising oxides of silicon and iron to a substantially closed furnace. Heat is supplied to the furnace by striking a direct current arc between a cathode electrode and an anode functional hearth. In a preferred embodiment of the present invention, the cathode electrode is hollow and feed to the substantially closed furnace is through the hollow electrode.
Kuhlman, U.S. Pat. No. 3,215,522, issued Nov. 2, 1965, describes a process for producing silicon metal-bearing alloys in an electric furnace. The process involves packing a mixture of silica, alloying ingredients such as reducible metal compounds or reduced metal, and a carbonaceous reducing agent around at least one hollow carbonaceous electrode. The feed to the furnace is separated into coarse and fine materials, with the fine material being added to the process through the hollow electrode and the coarse material being added to the furnace through an open top. The process described by Kuhlman uses a submerged-arc to supply heat to the furnace burden and effect smelting.
Goins et al., U.S. Pat. No. 4,865,643, issued Sep. 12, 1989, describes electrometallurgical processes for producing elemental silicon and silicon alloys in a furnace using a hollow direct current electrode as a heat source. The furnaces described by Goins et al. have open-tops. Goins et al. teach creating a bed of a carbonaceous reducing agent within the hollow electrode. Silicon monoxide containing off-gas from the smelting process is drawn through the hollow electrode and the silicon monoxide is reduced by the carbonaceous reducing agent to silicon.
Arvidson et al., U.S. Pat. No. 5,009,703, issued Apr. 23, 1991, describes a process for preparing silicon metal and silicon metal alloys in a substantially closed, direct current, submerged-arc furnace.
Dosaj et al., U.S. Pat. No. 4,898,712, issued Feb. 6, 1990, describe a process for preparing ferrosilicon in a closed two-stage reduction furnace. In the described process, carbon monoxide released as a result of the smelting process occurring in the first stage of the furnace is used to prereduce higher oxides of iron contained in the second stage of the furnace. The reduced oxides of iron are then used as a feed to the first stage of the furnace. Dosaj et al. teach that the heat provided to the furnace can be by means of an open or submerged graphite electrode connected to an alternating current or direct current power source. Dosaj et al. teach that iron oxide containing ores or their tailings can be used as a feed to the furnace.
Various embodiments of the present invention offer many of the following advantages over the prior art. First, the use of a substantially closed furnace reduces emission of oxides such as silicon monoxide and carbon monoxide to the environment. Second, the use of a substantially closed furnace reduces venting of fines from the furnace and increases feed utilization. Third, the use of a direct current power source reduces both power consumption and electrode consumption. Fourth, the use of a hollow electrode allows fines to be fed directly to the reaction zone of the furnace, facilitating the smelting process. Finally, the ability of the described furnace configuration to smelt fines allows the use of low cost feed materials such as coke breeze and tailings from iron ore refining.